This application claims priority to an application entitled xe2x80x9cMETHOD FOR FABRICATING HIGH-PURITY SILICA GLASS USING SOL-GEL PROCESSINGxe2x80x9d filed with the Korean Industrial Property Office on Dec. 31, 1999 and there duly assigned Ser. No. 99-68274.
1. Field of the Invention
The present invention relates generally to a method for fabricating silica glass, and in particular, to a method for fabricating high-purity silica glass using a sol-gel process.
2. Description of the Related Art
Silica glass, which is a raw material of high-purity glass articles, has been useful for optical devices, such as optical fiber and photomask for a semiconductor. Silica glass is generally manufactured using a natural quartz process, synthetic quartz process, or sol-gel process.
The sol-gel processing is a technique of securing high-purity silica glass through the provision of high purity materials as the starting materials. When the sol-gel process is used as a liquid phase process, high productivity can be achieved and the composition of the product can be controlled easily compared to other processes. Also, the sol-gel process is economical because almost all steps except sintering are carried out at a low temperature.
A fabrication method of silica glass using the sol-gel processing is disclosed in detail in U.S. Pat. No. 5,240,488 under the title, xe2x80x9cManufacture of vitreous silica product via a sol-gel process using a polymer additive.xe2x80x9d
In the fabrication of silica glass using the sol-gel processing, there are various factors including temperature, composition, pressure, acidity and solvency which have an adverse effect in the transition of sol to gel and the strength of the gel. To overcome the problem, an attempt has been made on a method for securing the flexibility and strength of the gel while preventing cracks during the drying process.
As disclosed in U.S. Pat. No. 5,240,488, the fabrication method of silica glass using the sol-gel processing involves dispersing the fumed silica in an alkaline region to form a sol, then adding the polymer and gelation agent to ultimately produce the final silica glass. This method helps to prevent some of the cracks in the gel during the drying step and reduces the residual carbon in the final product when an organic binder is added, thus allowing the production of a fairly large silica glass. However, there is still a high chance of cracking in the gel during the manufacturing process because the gel has low strength.
In other instances, the sol-gel processing involves the addition of the fluorine compound so as to reduce the refractivity and the heat expansion coefficient of silica as well as the viscosity of the silica glass at a high temperature. The fluorine compound is an acidic substance used as a gelation agent for a polymeric sol or a silica particulate sol, which are both dispersed in an acidic region with a low hydrogen ion content. However, the fluorine compound is not used as the gelation agent in the sol despersed in the alkaline region because the existing fluorine compound tends to lower the hydrogen icon content of the sol and causes unwanted rapid gelation.
It is, therefore, an object of the present invention to provide a method for fabricating high-purity silica glass using a sol-gel process in which a fluorine compound is added to provide bar-shaped silica glass with minimized cracks during the drying step and thus improves the plasticity.
It is another object of the present invention to provide a method for fabricating high-purity silica glass using a sol-gel process which allows the production of a particulate sol containing the fluorine compound even in the alkaline region so as to reinforce the wet gel, thereby minimizing the formation of cracks in the drying step.
Accordingly, the method for fabricating high-purity silica glass using a sol-gel processing includes the steps of: (a) mixing deionized water with a fluorine compound and a dispersion agent to prepare an aqueous premix solution; (b) mixing the aqueous premix solution with a famed silica; (c) mixing the resulting mixture to form a dispersed sol; (d) aging the dispersed sol at the ambient temperature to stabilize silica particles; and, (e) removing air voids from the sol and adding a gelation agent.